Is it normal for me to distance myself from God and for my relationship with Him to deteriorate after debating atheists and apostates, and can I return to what I was, or even better?

Satan is keen to lead the children of Adam astray, and if he finds them turning to Allah, he strives to repel them. Satanic whispers afflict everyone who turns to Allah, and the servant must stand firm, be patient, and adhere to dhikr (remembrance of Allah) and prayer so that Satan's treachery may be averted from him.

One must arm himself with sacred knowledge (Islamic sciences) to be able to engage in da'wah (inviting to Islam), for confronting the arguments of disbelievers and apostates without knowledge is dangerous. As for the feeling of regression, it may be a period of languor after striving hard in worship.